Merge branch 'compose-and-send': composing and sending mail

Item 123, built over 2026-08-20 to 2026-08-23 in thirteen tasks against
docs/superpowers/specs/2026-08-20-compose-and-send-design.md.

The application writes mail now. A composer window per message, markdown as
the body, drafts autosaving into the account's Maildir, and sending through a
per-account command on stdin rather than any network protocol of this
program's own. A countdown with an Undo stands between pressing Send and the
command running.

Two things came in alongside it. The notmuch auto-tagging hooks moved here
from the retiring `mailctl` project and learned that mail this application
files itself never arrived, so sent mail and drafts stop appearing in the
inbox. And the v1/v2 language is retired: semver on the user-visible surface
is the rule, and those labels described a split that composing made obsolete.

Hand tested against a fake send command rather than a real one, deliberately:
New, Reply and Forward all produce correct messages, a forwarded attachment
survives intact, and the sent copy is filed. That testing found the two
defects fixed on this branch, and both were invisible to the suite: a composer
orphaned by quitting the main window, and every sent message tagged `inbox`.

Twenty-two defects were found in the plan document's own draft code while
building it, which is why CLAUDE.md says to treat every code block in a plan
as a draft.

1 files changed, 110 insertions, 0 deletions

diff --git a/src/markdownrenderer.cpp b/src/markdownrenderer.cpp
new file mode 100644
index 0000000..7158981
--- /dev/null
+++ b/src/markdownrenderer.cpp
@@ -0,0 +1,110 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+// cmark-gfm's headers are C and carry no Qt interaction, so the gmime
+// include-order rule does not apply here. They still go first, for consistency
+// with mimeparser.cpp.
+#include <cmark-gfm.h>
+#include <cmark-gfm-core-extensions.h>
+
+#include "markdownrenderer.h"
+
+#include <QByteArray>
+
+#include <cstdlib>
+
+namespace {
+
+/// The extensions this application enables, by cmark-gfm's own names.
+///
+/// `table` is absent deliberately, not by oversight: tables render badly
+/// across mail clients regardless of who generates them. `tagfilter` is absent
+/// because safe mode (see below) already suppresses raw HTML wholesale, which
+/// is the stronger measure.
+const char *const kExtensions[] = { "autolink", "strikethrough", "tasklist" };
+
+}  // namespace
+
+QString MarkdownRenderer::toHtml(const QString &markdown)
+{
+    if (markdown.isEmpty())
+        return {};
+
+    // Idempotent, and a hash lookup after the first call. The function-local
+    // static makes the FIRST call thread-safe: cmark-gfm's registry carries no
+    // once-guard of its own, so two threads racing the first call would tear
+    // it. Today's only caller is on the UI thread; this costs nothing and
+    // removes the trap before a worker-thread caller finds it.
+    static const bool registered = [] {
+        cmark_gfm_core_extensions_ensure_registered();
+        return true;
+    }();
+    Q_UNUSED(registered)
+
+    // CMARK_OPT_DEFAULT is 0, and CMARK_OPT_SAFE is a NO-OP in cmark-gfm 0.29:
+    // safe mode has been the default since that release, and the flag is kept
+    // only for API compatibility with code written against older versions.
+    // The real requirement is that CMARK_OPT_UNSAFE must never be set. Under
+    // safe mode a raw <script> block is replaced with an HTML comment
+    // placeholder, and a link whose scheme is not in the allowed set
+    // (javascript:, vbscript:, file:, and data: except a few safe image
+    // types) is replaced with an empty href. Measured against
+    // cmark-gfm-0.29.0.gfm.13 on 2026-08-20: rendering the same script tag and
+    // a javascript: link under OPT_DEFAULT alone, under OPT_DEFAULT|OPT_SAFE,
+    // and under OPT_UNSAFE shows the first two behave identically and
+    // suppress both, while OPT_UNSAFE leaks both verbatim into the output.
+    // OPT_SAFE is kept anyway, both as a statement of intent and in case a
+    // future cmark-gfm release makes it meaningful again; do not read its
+    // presence as the mechanism actually doing the suppressing.
+    const int options = CMARK_OPT_DEFAULT | CMARK_OPT_SAFE;
+
+    cmark_parser *parser = cmark_parser_new(options);
+    if (!parser)
+        return {};
+
+    for (const char *name : kExtensions) {
+        // A missing extension is a broken installation rather than a
+        // condition to handle: the library was found by CMake. Skipping it
+        // degrades to plain CommonMark rather than crashing.
+        if (cmark_syntax_extension *extension = cmark_find_syntax_extension(name))
+            cmark_parser_attach_syntax_extension(parser, extension);
+    }
+
+    const QByteArray utf8 = markdown.toUtf8();
+    cmark_parser_feed(parser, utf8.constData(), static_cast<size_t>(utf8.size()));
+
+    cmark_node *document = cmark_parser_finish(parser);
+    if (!document) {
+        cmark_parser_free(parser);
+        return {};
+    }
+
+    // The extension list must be passed to the renderer as well as to the
+    // parser. Passing nullptr here parses the tasklist correctly and then
+    // renders it as a plain list item, which looks like the extension never
+    // worked.
+    char *html = cmark_render_html(document, options,
+                                   cmark_parser_get_syntax_extensions(parser));
+    const QString result = html ? QString::fromUtf8(html) : QString();
+
+    free(html);
+    cmark_node_free(document);
+    cmark_parser_free(parser);
+
+    return result;
+}